1. Field of the Invention
The present invention relates to a method of forming an electrode pattern of a surface acoustic wave device, for example, a SAW (Surface Acoustic Wave) filter.
2. Description of the Related Art
As a method of forming an electrode pattern of a surface acoustic wave device, the method described below is conventionally used. First, a resist layer is formed on a top surface of a piezoelectric substrate made of lithium tantalate (LiTaO3) or lithium niobate (LiNbO3), and a photomask is formed on the resist layer. Next, exposure is performed by irradiating the top surface of the piezoelectric substrate with ultraviolet rays referred to as xe2x80x9cg-line (wavelength 436 nm)xe2x80x9d rays or xe2x80x9ci-line (wavelength 365 nm)xe2x80x9d rays from above, and then development is performed to form a resist pattern. Then, a conductor film of Al is deposited on the top surface of the piezoelectric substrate, and then the resist remaining on the top surface of the piezoelectric substrate is removed together with the conductor film formed on the resist pattern. As a result, an electrode pattern is formed in the portion of the top surface of the piezoelectric substrate which is not covered with the resist pattern.
However, in the above-described electrode forming method, as shown in FIG. 11, ultraviolet rays applied to the top surface of a piezoelectric substrate 1 through a transmission hole 6 of a photomask 5 during exposure mostly pass through a resist layer 2 and enter the piezoelectric substrate 1. The ultraviolet rays entering the piezoelectric substrate 1 are reflected diffusely by the bottom surface of the piezoelectric substrate 1, and the diffused reflection ultraviolet rays again return to the top surface of the piezoelectric substrate 1 to reach a portion of the resist layer, which should not be exposed, thereby causing the problem of exposing the portion of the resist layer to light. Particularly, when the bottom surface of the piezoelectric substrate is roughened, diffused reflection is increased. Consequently, the resist pattern formed by subsequent development has a shape that is different from an initial desired shape, thereby causing the problem of failing to obtain a desired electrode pattern. Particularly, it is known that when the light width or space width of the resist pattern is about 1 xcexcm or less, this problem becomes significant.
As a method for solving the problem, Japanese Unexamined Patent Application Publication No. 10-233641 discloses an electrode pattern forming method. This method includes previously forming an anti-reflection film on the bottom surface of the piezoelectric substrate, and then forming an electrode pattern. Specifically, an organic polymer film containing a dye having absorptivity for the wavelength of ultraviolet irradiation light is used as the anti-reflection film. As shown in FIG. 12, when ultraviolet rays applied to the resist layer 2 through the transmission hole 6 of the photomask 5 during exposure partially pass through the resist layer 2 and enter the piezoelectric substrate 1, the ultraviolet rays are absorbed by the anti-reflection film 3 disposed on the bottom surface of the piezoelectric substrate 1 to prevent reflection from the bottom surface of the piezoelectric substrate 1. Therefore, the occurrence of abnormalities in the shape of the resist pattern is prevented. A back conductor film 4 formed below the anti-reflection film 3 functions to cause any static electricity produced by heat treatment during the exposure step to escape to the outside.
However, the above-described electrode pattern forming method requires the additional step of forming the anti-reflection film which complicates and extends the manufacturing process, thereby increasing the manufacturing cost. Also, the piezoelectric substrate and the anti-reflection film have different refractive indexes, and thus light reflected from the interface cannot be completely removed to inevitably cause reflection of a slight ultraviolet ray. Therefore, the occurrence of an abnormality in the shape of the resist pattern cannot be completely prevented.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of forming an electrode pattern of a surface acoustic wave device which is capable of easily and completely preventing the occurrence of an abnormality in the shape of a resist pattern.
In accordance with a preferred embodiment of the present invention, a method of forming an electrode pattern of a surface acoustic wave device includes the steps of forming a resist layer on a top surface of a piezoelectric substrate, exposing the resist layer to ultraviolet rays through a photomask provided above the top surface of the piezoelectric substrate to form a resist pattern, forming a conductor film over the piezoelectric substrate, and removing the resist pattern, wherein the step of exposing the resist layer is performed by using ultraviolet rays having a wavelength that is completely absorbed into the piezoelectric substrate without reaching the bottom surface of the piezoelectric substrate.
By using the above-described method of forming an electrode pattern of a surface acoustic wave device, the electrode pattern can be obtained according to a desired design because the ultraviolet rays are absorbed into the piezoelectric substrate without reaching the bottom surface of the piezoelectric substrate. Also, the conventional step of forming an anti-reflection film on the bottom surface of the piezoelectric substrate is not required, which decreases the time and costs required for the manufacturing process.
Furthermore, exposure to ultraviolet rays with a short wavelength permits the formation of a resist pattern having a line width or space width of about 0.25 xcexcm or less, thereby further miniaturizing the surface acoustic wave device. Also, the focal depth of ultraviolet ray in exposure is inversely proportional to the wavelength of the ultraviolet ray. Thus, in forming a resist pattern having the same line width or space width as a conventional value, the focal depth is significantly increased to extend the exposure area of a stepper, which is an expensive device, thereby improving throughput and decreasing manufacturing cost.
In the method of forming the electrode pattern of the surface acoustic wave device of preferred embodiments of the present invention, for example, exposure is preferably performed through a photomask having a slightly wider opening than a desired resist line, and the resist pattern is preferably formed to have a reverse tapered sectional shape.
By using the photomask having the slightly wider opening than the desired resist line as described above, the resist pattern can be controlled to a desired width with a relatively small exposure. Also, when the resist pattern is formed to have a reverse tapered sectional shape, a separating solution used for separating the resist pattern easily permeates to the bottom (the interface with the piezoelectric substrate) of the resist pattern, improving the separating property of the resist pattern.
In the method of forming the electrode pattern of the surface acoustic wave device of preferred embodiments of the present invention, for example, the piezoelectric substrate preferably includes a lithium tantalate substrate and has a thickness of about 0.2 mm or greater, and the wavelength of the ultraviolet ray is preferably about 265 nm or less.
As described above, when the lithium tantalate substrate having a thickness of about 0.2 mm or greater is used as the piezoelectric substrate, and ultraviolet rays with a wavelength of about 265 nm or less is used as the ultraviolet rays for exposure, the ultraviolet rays are absorbed into the piezoelectric substrate without reaching the bottom surface of the piezoelectric substrate, and thus, the electrode pattern can be obtained according to a desired design.
In the method of forming the electrode pattern of the surface acoustic wave device of preferred embodiments of the present invention, for example, the piezoelectric substrate preferably includes a lithium niobate substrate and has a thickness of about 0.2 mm or greater, and the wavelength of the ultraviolet rays is preferably about 305 nm or less.
As described above, when the lithium niobate substrate having a thickness of about 0.2 mm or greater is used as the piezoelectric substrate, and ultraviolet rays with a wavelength of about 305 nm or less is used as the ultraviolet ray for exposure, the ultraviolet rays are absorbed into the piezoelectric substrate without reaching the bottom surface of the piezoelectric substrate, and thus the electrode pattern can be obtained according to a desired design.
In the method of forming the electrode pattern of the surface acoustic wave device of preferred embodiments of the present invention, for example, a KrF or ArF excimer laser light is preferably used as the ultraviolet ray, and a chemical amplification-type negative photoresist is preferably used as the resist.
As a means for realizing the ultraviolet ray with a wavelength of about 265 nm or less, a KrF excimer laser exposure device and an ArF excimer laser exposure device can be used. A combination of such an excimer laser and a chemical amplification-type negative photoresist enables the formation of the resist pattern having a line width or space width of about 0.25 xcexcm or less.
The method of forming the electrode pattern of the surface acoustic wave device of preferred embodiments of the present invention is effective, for example, for a case in which a resist pattern has a line width or space width of about 1 xcexcm or less.
As described above, when the electrode pattern having a line width or space width of about 1 xcexcm or less is formed by a liftoff method, a desired resist pattern cannot be obtained due to the influence of light reflected by the rear surface of the piezoelectric substrate. Therefore, the method of forming the electrode pattern of the surface acoustic wave device of preferred embodiments of the present invention is effective for a case in which an electrode pattern having a line width or space width of about 1 xcexcm or less is formed by the liftoff method.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.